Liquid dispensing guns are used in industry for a variety of applications. Such applications might include the dispensing of adhesives to a carton or the like, which adhesives might include hot melt, atmospheric setting, ultraviolet setting, temperature based curing adhesives, and self curing epoxies, among others; the dispensing of paint to an ornament or decorative object; the dispensing of lubricants to various parts of mechanisms or machines; and the dispensing of sealants to a wide variety of articles, among other applications. It is common to have such liquid dispensing guns operatively connected to a robotic arm or to an X-Y-Z table. In either case, the motion of the dispensing gun with respect to the article having liquid deposited thereon is independently controlled in each of the X, Y, and Z axes, and can be determined at any time or point along the path of the dispensing gun. The speed of the dispensing nozzle across the receiving surface is the vectorial sum of the X, Y, and Z components of the speed and may be calculated using the equation: EQU surface speed=(speed in X direction.sup.2 +speed in Y direction.sup.2 +speed in Z direction.sup.2).sup.1/2
The dispensing guns for each particular application are designed so as to be specifically suited to that application. Each type of dispensing gun uses a valve, such as a needle valve, located within the nozzle of the dispensing gun at a dispensing output opening therein to open and close the dispensing output. The valve means is moveable, typically by way of an air actuated solenoid, between a full flow position where the liquid contained in the dispensing gun is dispensed through the dispensing output opening in the nozzle, and a flow precluding position where the valve means is intimately engaged against a co-operating seat so as to preclude the flow of liquid from the nozzle. In the full flow position, the needle valve contacts a back stop, thus defining the full flow position of the needle valve.
The flow rate of the fluid from such dispensing guns is selected depending on the particular application, the properties of the particular liquid being dispensed, and so on. It is important to select a proper flow rate as it is important to apply such liquids as a constant volume per unit length of liquid dispensed, with any more than a very minor variation being generally unacceptable. Most dispensing guns have manually selectable flow rate that is set by way of a hand operated control mechanism that positions the back stop so as to define the full flow position of the needle valve. This full flow position is typically set only once for a given application. A selected flow rate is, by definition, a constant volume of liquid flow per unit time. If the nozzle of the dispensing gun travels across the receiving surface at a constant speed, a corresponding constant volume of liquid will be dispensed per unit length of liquid dispensed along the receiving surface. However, if the nozzle of the dispensing gun does not travel across the receiving surface at a constant speed, the volume of liquid dispensed per unit length of liquid dispensed along the receiving surface will vary proportionately with the speed of travel of the nozzle across the receiving surface.
It is very important to be able to maintain a constant application of the liquid being dispensed per unit length of liquid dispensed along the receiving surface so as to preclude over-dispensing or under-dispensing. The amount of the liquid dispensed along an application path on a receiving surface can change as one or more of several related parameters change, such parameters including the speed of the nozzle of the dispensing gun with respect to the receiving surface, the temperature of the liquid, the viscosity of the liquid, the narrowing of the dispensing opening of the nozzle due to partial clogging, and so on. For instance, if the nozzle of the dispensing gun tracks a square corner, the speed of the nozzle across the receiving surface near or at the corner is less than the targeted predetermined speed of the nozzle across the receiving surface. In this instance, since the actual dispensing rate per unit time of the liquid from the nozzle does not change, an increase occurs in the amount of liquid dispensed per unit length of liquid dispensed at the corner--in other words, excess liquid is dispensed at the corner. Further, as the temperature of the liquid being dispensed rises, the viscosity may either fall or rise, depending on the type of liquid, which therefore causes a corresponding change in the amount of flow of liquid from the nozzle per unit time, and a corresponding change in the amount of liquid dispensed per unit length of liquid dispensed along the receiving surface. Also, as the dispensing of the liquid continues, it is possible that the nozzle can partially clog, thus reducing the amount of liquid dispensed per unit time, thus reducing the amount of liquid dispensed per unit length of liquid dispensed along the receiving surface. In any event, any substantial change in amount of liquid dispensed per unit length of liquid dispensed along the receiving surface is unacceptable.
It can be seen that it is necessary to control the rate of flow of liquid from a nozzle per unit time in order to regulate the amount of liquid per dispensed unit length of liquid dispensed along the receiving surface. For instance, as the nozzle traverses a right angled corner, the rate of liquid dispensed from the nozzle per unit time must be slowed in proportion to the speed of the nozzle across the receiving surface. This same principle also applies to a rounded corner. In such instance, the speed of the nozzle with respect to the receiving surface may have to be calculated vectorially on a continuing and instantaneous basis using the equation: EQU surface speed=(speed in X direction.sup.2 +speed in Y direction.sup.2 +speed in Z direction).sup.1/2
Further, as the temperature of the liquid being dispensed rises, and the viscosity correspondingly drops, the amount of liquid flowing from the nozzle per unit time may increase, even though the size of the opening in the nozzle has not increased. Accordingly, the size of the opening in the nozzle may have to be correspondingly decreased. Further, as the nozzle becomes partially clogged through continuing use, it may be necessary to further open the valve within the nozzle so as to maintain a constant flow of liquid therefrom per unit length of liquid dispensed along the receiving surface.
Another problem with such prior art liquid dispensing guns is that the air actuated solenoid that operates the needle valve tends to open and close the valve quite abruptly. Accordingly, it is typical to have a sudden, but short lived, overflow of liquid shoot forth from the dispensing gun when the valve is first opened, which is highly undesirable, if not unacceptable.